This invention relates generally to a method of high resolution electrostatic transfer of a high density image to a nonporous, nonabsorbent conductive receiving surface. More specifically, it pertains to the method of transfer and the method of creating a latent image on an electrostatically imageable surface that may be repeatedly used to produce high resolution and high density images on printed circuit boards.
The production of conductive wiring patterns on an insulating substrate employing a dry film resist by use of photoimaging and other techniques to produce a printed circuit board typically employs a five step process. Regardless of whether a tenting-method or a hole-plugging method is employed, the five distinct steps have included laminating or coating a photosensitive dry film resist on at least one conductive surface of an insulating substrate, forming a wiring pattern on the dry film resist by use of artwork or a phototool and xxposing the dry film resist to actinic radiation through the transparent areas of the phototool, developing the circuit board by removing the unexposed portions of the negative working dry film resist, etching the conductive substrate from the circuit board in all non-imaged areas not beneath the desired conductive wiring pattern which is still covered with the dry film resist, and finally stripping or removing the dry film resist covering the desired wiring pattern from the non-etched portions of the conductive substrate. This five step process must be repeated for each circuit board produced.
During the exposure step in the standard dry film process, sufficient radiation exposure levels and exposure times are desired to produce straight sidewalls in the dry film resist that are the result of a pattern of the cross-linking of polymers in the dry film. These straight sidewalls should be normal to the conductor surface. Practically, however, in the standard negative working dry film photoresist print and etch process either underexposure occurs, producing a sidewall edge that undercuts the desired resist pattern, or overexposure occurs, producing a sidewall edge in the dry film photoresist that increases the width of the dry film photoresist at the base of the resist and the surface of the conductor causing a foot. Both of these conditions vary the width of the ultimate conductive pattern from that which is desired, beyond the planned and engineered tolerance or overage of the line widths in the conductor surface.
The development step during this process ideally should develop away the unexposed negative working dry film resist to produce an edge in the dry film resist on the conductor surface that is equal in width to the pattern on the phototool and normal to the conductor surface. Practically, however, either underdevelopment or overdevelopment of the dry film photoresist occurs. Underdevelopment produces a buildup of resist residue in the sidewall zone or developed channels that is sloped toward the adjacent sidewall resulting in smaller spaces between the adjacent lines than is desired. When overdevelopment occurs the unexposed film resist edge is undercut, producing larger than desired spacing between adjacent lines. Additionally, there is the potential for some rounding at the top of the resist surface sidewall edges.
This inability to accurately reproduce the phototool in the dry film resist affects the fine line resolution and reproduction characteristics of the reproduced circuit pattern. As circuit boards have become more complex and stacking of multiple boards has become prevalent, the need for higher density, finer resolution circuit patterns has evolved. Resolution has been viewed as the ability to reliably produce the smallest line and space between adjacent lines that can be reliably carried through the aforementioned five step processing. The thinness or smallness of the lines that can survive development and the narrowness of the gap or space between the adjacent lines in the circuit pattern have led to fine line resolution and reproduction standards in the printed circuit board industry calling for about 3.1 mil line and space dimensions or the development of about 6.3 line pairs per millimeter. These standards are used to define the desired density of the circuit board.
The attempt to apply the principles of xerography to transfer developed electrostatic latent images from a photoconductor's electrostatically imaged surface to a receiver surface with high resolution and high density images has encountered difficulty. The major source of this difficulty stems from the fact that circuit boards consist of a nonporous or nonabsorbent substrate, such as metal, like copper, or a plastic, like the polyester film sold under the tradename of MYLAR. This nonporous and nonabsorbent receiving surface causes the image being transferred, especially when attempted with a liquid toner, to become distorted or "squished".
Xerographic techniques solved the problem of transferring an image to absorbent receiving surfaces, such as paper, by transferring the images formed by L toner particles across a gap. The gap has either been an air, or a combination air-liquid, gap. Attempts to translate this gap transfer technology to nonporous substrates, however, resulted in image "squish" and the realization that the gap space and the voltage must be carefully controlled to produce an acceptable transferred toner image with the proper resolution and density. If the voltage and the gap space or distance between the photoconductor or the electrostatically imageable surface and the conductive receiving surface are not carefully controlled, electrical arcing across the gap will occur. This can- cause pin-holes in the transferred toner image by permanently damaging the electrostatically imageable surface. This is especially significant in print and etch applications used to manufacture printed circuit boards.
Also, it has been found with nonporous receiving substrates that both the photoconductor or electrostatically imageable surface and the receiving conductive surface must be stationary at the point of transfer of the toner image to achieve a transferred image of high resolution.
An additional problem is presented in transferring the developed latent image electrostatically to a nonabsorbent substrate, such as copper. The metal or copper surface forming the conductive receiving surface, as well as the electrostatically imageable surface, is uneven so that the spacing between the electrostatically imageable surface and the conductive receiving surface must be sufficient to avoid contact between the uneven surfaces of the photoconductor and the conductive receiving surface.
These problems are solved in the process of the present invention by providing a method of making a transfer of a developed electrostatic latent image from an electrostatically imageable surface across a liquid-filled gap to a conductive nonabsorbent, nonporous receiving surface to produce multiple printed circuit boards with a desired conductive pattern from a single persistent latent image. The electrostatically imageable surface may be either a photoconductor or a permanent master.